The Death Domain Kinase RIP1 Is Essential for Tumor Necrosis Factor Alpha Signaling to p38 Mitogen-Activated Protein Kinase

Abstract

The cytokine tumor necrosis factor alpha (TNF-α) stimulates the NF-κB, SAPK/JNK, and p38 mitogen-activated protein (MAP) kinase pathways by recruiting RIP1 and TRAF2 proteins to the tumor necrosis factor receptor 1 (TNFR1). Genetic studies have revealed that RIP1 links the TNFR1 to the IκB kinase (IKK) complex, whereas TRAF2 couples the TNFR1 to the SAPK/JNK cascade. In transfection studies, RIP1 and TRAF2 stimulate p38 MAP kinase activation, and dominant-negative forms of RIP1 and TRAF2 inhibit TNF-α-induced p38 MAP kinase activation. We found TNF-α-induced p38 MAP kinase activation and interleukin-6 (IL-6) production impaired in rip1 ^−/− murine embryonic fibroblasts (MEF) but unaffected in traf2^−/− MEF. Yet, both rip1 ^−/− and traf2 ^−/− MEF exhibit a normal p38 MAP kinase response to inducers of osmotic shock or IL-1α. Thus, RIP1 is a specific mediator of the p38 MAP kinase response to TNF-α. These studies suggest that TNF-α-induced activation of p38 MAP kinase and SAPK/JNK pathways bifurcate at the level of RIP1 and TRAF2. Moreover, endogenous RIP1 associates with the MAP kinase kinase kinase (MAP3K) MEKK3 in TNF-α-treated cells, and decreased TNF-α-induced p38 MAP kinase activation is observed in Mekk3 ^−/− cells. Taken together, these studies suggest a mechanism whereby RIP1 may mediate the p38 MAP kinase response to TNF-α, by recruiting the MAP3K MEKK3.

ACKNOWLEDGMENTS

This work was supported by an American Cancer Society Research Project grant and National Institutes of Health grant GM61298 to M.A.K. M.A.K. is also the recipient of a Sidney Kimmel Cancer Scholar Award.